1. Field of the Invention
The present invention relates to a method of driving a mask stage and a method of mask alignment. More particularly, the present invention relates to a method of driving a stage, which is suitably applied to a case where a reticle-side stage is driven in a scan direction in a slit-scan exposure type projection exposure apparatus, and a method of mask alignment in the projection exposure apparatus.
2. Related Background Art
When a semiconductor element, a liquid crystal display element, a thin film magnetic head, or the like is manufactured in a photolithography process, a projection exposure apparatus for transferring a pattern on a photomask or a reticle (to be generally referred to as a xe2x80x9creticlexe2x80x9d hereinafter) onto a substrate (a wafer, glass plate, or the like) coated with a photosensitive material is used.
As a conventional projection exposure apparatus, a step-and-repeat type reduction projection exposure apparatus (stepper) for sequentially exposing a pattern image on a reticle onto each of shot areas by sequentially moving the shot areas of a wafer into an exposure field of a projection optical system is popularly used.
In recent years, since patterns on semiconductor devices or the like tend to be miniaturized, it is required to increase the resolution of a projection optical system. For this reason, in order to increase the resolution, a technique for decreasing the wavelength of exposure light, a technique for increasing the numerical aperture of the projection optical system, and the like have been examined. However, with either technique, it becomes difficult to maintain high accuracy of imaging performance (a distortion, curvature of field, and the like) on the entire exposure field when an exposure field as large as that in the prior art is to be assured. For this reason, an apparatus which is currently reconsidered its use is a so-called slit-scan exposure type projection exposure apparatus.
In the slit-scan exposure type projection exposure apparatus, a pattern on a reticle is exposed onto a wafer, wherein the reticle and wafer are being synchronously scanned relative to a rectangular or arcuated illumination area (to be referred to as a xe2x80x9cslit-like illumination areaxe2x80x9d hereinafter).
Therefore, when a pattern with the same area as that in the stepper system is to be exposed onto a wafer, the exposure field of the projection optical system in the slit-scan exposure system can be set to be smaller than that in the stepper system. As a result, accuracy of imaging performance in the exposure field may be improved.
The mainstream of the conventional reticle size is 6xe2x80x3, and the mainstream of the projection magnification of the projection optical system is x⅕. However, as the area of the circuit pattern of, e.g., a semiconductor element increases, the 6 xe2x80x3 reticle cannot serve its purpose at the x⅕ magnification. For this reason, a projection exposure apparatus in which the projection magnification of the projection optical system is changed to, e.g., xxc2xc must be designed. In order to cope with such an increase in area of a pattern to be transferred, the slit-scan exposure system is advantageous.
In a projection exposure apparatus of this type (stepper), a reticle must be aligned in advance on a reticle stage. For this purpose, a reticle alignment device is arranged on a reticle mark on the reticle. Such a reticle alignment device is disclosed in U.S. Pat. No. 4,710,029. In an alignment system disclosed in U.S. Pat. No. 4,710,029, light reflected by an alignment mark on a reticle is incident on a sensor via a vibration mirror and a slit. When the output from the sensor is synchronously detected by a driving signal of the vibration mirror, the position of the alignment mark relative to a slit is detected. The position of the alignment mark is detected based on a signal from the sensor in the alignment system, and the reticle is moved by a servo system, so that the alignment mark accurately coincides with the slit. As a result, alignment of the reticle with respect to the apparatus main body is executed.
In such a slit-scan exposure system, when the moving path of the reticle stage for driving a reticle is curved with respect to a desired path (for example, the moving path has a predetermined curvature with respect to a desired linear path), each shot area on a wafer undesirably has an intra-shot distortion according to the curve (curvature) of the moving path of the reticle stage. Furthermore, when the characteristics of an intra-shot distortion vary from one exposure apparatus to another, such a variation results in a matching error between different layers on the wafer. When the reticle stage is controlled by a method of measuring the position of the reticle stage by interfering light components reflected by a stationary mirror and a movable mirror provided to the reticle stage using an optical interferometer, such a curve of the path of the reticle stage is caused by a curve of the movable mirror.
The present invention has been made in consideration of the above situation, and has as its object to provide a method of driving a stage, which can prevent generation of an intra-shot distortion even when a movable mirror provided to a stage at the side of reticle (mask) has a curve in a slit-scan exposure type exposure apparatus.
In order to achieve the above object, according to the first invention, there is provided a method of driving a mask stage using the mask stage which mounts a mask formed with a predetermined pattern and is movable in a predetermined scan direction, a movable mirror which is arranged on the mask stage and has a reflection surface substantially parallel to the scan direction, measurement means for measuring a coordinate position, in a direction perpendicular to the scan direction, of the mask stage by radiating a measurement beam into the movable mirror, a substrate stage which mounts a photosensitive substrate and is movable in a direction substantially parallel to the scan direction, an illumination system for illuminating a predetermined area on the mask with illumination light, a projection optical system for projecting the pattern on the mask onto the photosensitive substrate, and an exposure device for sequentially exposing the pattern on the mask onto the photosensitive substrate while synchronously scanning the mask stage and the substrate stage in the scan direction with respect to an optical axis of the projection optical system, comprising:
the first step of placing the mask on the mask stage;
the second step of calculating a curved amount of the movable mirror by measuring the coordinate position, in the direction perpendicular to the scan direction, of the mask stage by the measurement means while scanning the mask stage in the scan direction; and
the third step of moving the mask stage in the direction perpendicular to the scan direction to correct the curved amount of the movable mirror calculated in the second step when the mask stage is scanned in the scan direction with respect to the optical axis.
According to the second invention, there is provided a method of driving a mask stage using a mask guide which is formed with a guide portion extending in predetermined scan direction, the mask stage which is mounted on the mask. Guide to be movable in the scan direction, and mounts a mask formed with a predetermined pattern, a movable mirror which is attached to the mask stage, and has a reflection surface substantially parallel to the scan direction, measurement means for measuring a coordinate position, in a direction perpendicular to the scan direction, of the mask stage by radiating a measurement beam into the movable mirror, a substrate stage which is movable in the direction substantially parallel to the scan direction and mounts a photosensitive substrate, an illumination system for illuminating a predetermined area on the mask with illumination light, a projection optical system for projecting the pattern on the mask onto the photosensitive substrate, and an exposure device for sequentially exposing the pattern on the mask onto the photosensitive substrate while synchronously scanning the mask stage and the substrate stage in the scan direction with respect to an optical axis of the projection optical system, comprising the steps of:
calculating a curved amount of the movable mirror by measuring the coordinate position, in the direction perpendicular to the scan direction, of the mask stage by the measurement means by scanning the mask stage in the scan direction with reference to the mask guide; and
moving the mask stage in the direction perpendicular to the scan direction so as to correct the curved amount of the movable mirror when a transfer mask is scanned via the mask stage in the scan direction with respect to the predetermined shaped illumination area.
According to the first invention, since the curved amount of the movable mirror is measured with reference to the measurement mark provided to the mask, and the measured curved amount is corrected in exposure, even when the movable mirror provided to the mask stage on the side of the mask has a curve, generation of an intra-shot distortion at the substrate side can be prevented.
According to the second invention, since the curved amount of the movable mirror is measured with reference to the mask guide, when the straightness of the mask guide is good, the curve amount of the movable mirror can be quickly and easily measured, and the measured curved amount can be corrected in exposure.
In the slit-scan exposure type projection exposure apparatus as well, when a reticle is exchanged with another one, the new reticle must be aligned. However, in the slit-scan exposure system for driving a reticle in a predetermined direction with high accuracy during exposure, a reticle interferometer for monitoring the position of the reticle with high accuracy must be mounted. For this reason, it is difficult to assure larger driving strokes of the reticle in the X and Y directions and the rotational direction than those in a conventional stepper type projection exposure apparatus upon alignment of a reticle. Therefore, it is difficult to directly apply an alignment method used in the conventional stepper to the slit-scan exposure type projection exposure apparatus.
In general, the electron beam drawing error of a reticle mark with respect to the outer shape of a reticle is about xc2x10.5 mm to xc2x11 mm. In this case, when the reticle is aligned on the reticle stage with reference to its outer shape, if a pattern drawing area is inclined at a maximum inclination angle with respect to the outer shape of the reticle, the lateral shift amount of a laser beam from a reticle interferometer exceeds an allowable value of the lateral shift amount in a receiver of the interferometer. Therefore, it is difficult to completely correct the drawing error of a reticle in the conventional alignment method without causing a measurement error of the reticle interferometer.
Furthermore, an apparatus which mounts such a reticle interferometer can align a reticle at an arbitrary position with high accuracy by open-loop control. For this reason, an alignment method which can detect the position of a reticle mark at high speed by open-loop control must be developed in place of conventional closed-loop control (servo control) based on synchronous detection.
The present invention has been made in consideration of the above situation, and has as its object to provide an alignment method which can align a reticle (mark) at high speed with high accuracy in a slit-scan exposure type projection exposure apparatus.
In order to achieve the above object, according to the third invention, there is provided a method of aligning a mask with respect to a coordinate system on the side of a mask stage as pre-processing for exposing a pattern on the mask onto a photosensitive substrate using the mask stage which mounts the mask formed with a predetermined pattern and is movable in a predetermined scan direction, a substrate stage which mounts the photosensitive substrate and is movable in a direction substantially parallel to the scan direction, an illumination system for illuminating a predetermined illumination area on the mask with illumination light, a projection optical system for projecting the pattern on the mask onto the photosensitive substrate, observation means for observing a mark on the mask, and an exposure device for sequentially exposing the pattern on the mask onto the photosensitive substrate while synchronously scanning the mask stage and the substrate stage in the scan direction with respect to an optical axis of the projection optical system, comprising:
the first step of placing, as the mask, a mask formed with a first alignment mark having two linear patterns which cross each other, on the mask stage;
the second step of moving the two linear patterns in a direction to cross each other on the first alignment mark on the mask relative to an observation area of the observation means;
the third step of calculating a coordinate position, in the coordinate system on the side of the mask stage, of a crossing point of the two linear patterns of the first alignment mark by processing image data obtained by the observation means; and
the fourth step of aligning the mask to the coordinate system on the side of the mask stage on the basis of the coordinate position of the crossing point of the two linear patterns of the first alignment mark.
According to the fourth invention, there is provided a method of aligning a mask with respect to a coordinate system on the side of a mask stage as pre-processing for exposing a pattern on the mask onto a photosensitive substrate using the mask stage which mounts the mask formed with a predetermined pattern and is movable in a predetermined scan direction, a substrate stage which mounts the photosensitive substrate and is movable in a direction substantially parallel to the scan direction, an illumination system for illuminating a predetermined illumination area on the mask with illumination light, a projection optical system for projecting the pattern on the mask onto the photosensitive substrate, and an exposure device for sequentially exposing the pattern on the mask onto the photosensitive substrate while synchronizing scanning the mask stage and the substrate stage in the scan direction with respect to an optical axis of the projection optical system, comprising:
the first step of placing, as the mask, a mask formed with an alignment mark, on the mask stage; and
the second step of calculating a rotational angle of the mask with respect to the coordinate system on the side of the mask stage by calculating a coordinate position of the alignment mark, and when the rotational angle calculated in the second step exceeds a predetermined allowable value, the method further comprising:
the third step of unloading the mask from the mask stage;
the fourth of rotating the mask stage by a predetermined rotational angle in a direction of the rotational angle calculated in the second step; and
the fifth stage of placing the mask on the mask stage again, and rotating the mask stage in a direction opposite to the rotational direction in the fourth step.
According to the fifth invention, there is provided a method of aligning a mask with respect to a coordinate system on the side of a mask stage as pre-processing for exposing a pattern on the mask onto a photosensitive substrate using the mask stage which mounts the mask formed with a predetermined pattern and is movable in a predetermined scan direction, a substrate stage which mounts the photosensitive substrate and is movable in a direction substantially parallel to the scan direction, an illumination system for illuminating a predetermined illumination area on the mask with illumination light, a projection optical system for projecting the pattern on the mask onto the photosensitive substrate, and an exposure device for sequentially exposing the pattern on the mask onto the photosensitive substrate while synchronously scanning the mask stage and the substrate stage in the scan direction with respect to an optical axis of the projection optical system, comprising:
the first step of placing, as the mask, a mask formed with an alignment mark, on the mask stage; and
the second step of calculating a rotational angle of the mask with respect to the coordinate system on the side of the mask stage by calculating a coordinate position of the alignment mark, and when the rotational angle calculated in the second step exceeds a predetermined allowable value, the method further comprising:
the third step of rotating the mask stage in a direction opposite to the rotational angle calculated in the second step;
the fourth step of unloading the mask from the mask stage; and
the fifth stage of rotating the mask stage in a direction opposite to the rotational direction in the third step, and placing the mask on the mask stage again.
According to the third invention, when the mask stage is driven with respect to the observation area of the observation means so as to obliquely scan the mask, the coordinate position of the crossing point of the two linear patterns of the alignment mark on the mask can be measured by the open-loop control. Therefore, mask alignment can be realized at high speed with high accuracy.
According to the fourth and fifth inventions, when a mask is re-placed on the mask stage upon occurrence of a rotation error of the mask which poses a problem when a slit-scan exposure type mask stage is used, mask alignment can be realized at high speed with high accuracy. Also, strokes of the reticle stage upon alignment need not be increased, and length measuring means need not have any correction mechanism.